The invention relates to a Raster Output Scanner (ROS) system having an overfilled facet design and, more particularly, to an improved optical system for reducing the polygon facet-to-facet fast scan jitter.
Digital printers incorporating Raster Output Scanners use a rotating polygon as the scanning element to form modulated scan lines on the surface of a photosensitive medium. In a typical system, a beam, modulated according to an input video signal, is emitted from a light source such as a helium neon or a diode laser. The modulated light is directed through pre-polygon conditioning optics, onto the facets of a rotating polygon. The polygon rotates in the 3 to 30 krpm range, thus scanning the beam through a post-polygon optical system and imaging the laser spot as a scan line across the full process width of a photosensitive medium moving in a process direction. In prior ROS systems there are typically two scanning modes. In a first mode, pre-polygon conditioning optics incorporate an underfilled design; e.g. the light from the laser is either collimated, in the case of a diode laser, or expanded in the case of a gas laser, and collimated to a beam width in the fast scan direction that is smaller than the polygon facet, typically by a factor of approximately 3. The underfilled design has been generally preferred because of a high throughput efficiency and uniform illumination of the imaging facet. A second mode is the overfilled design where the light beam is collimated (laser diode) or expanded (gas laser) to a beam width in the fast scan direction that is larger than the polygon facet by a factor of 3 or so in the fast scan direction. In an overfilled design, the facet size required to produce a given spot size at the image medium is greatly reduced allowing many more facets to be accommodated on the same diameter polygon. This, in turn, permits the scan system to form more scan lines per second with a given polygon motor, or, alternatively, to permit the use of less powerful and less expensive polygon motor drives. The overfilled design has several disadvantages which have heretofore not been completely resolved. The throughput efficiency is relatively low (20%), compared to the 50% efficiency of the underfilled design, and the illumination of the imaging facet is not as uniform as the underfilled design. This illumination problem has been addressed by the techniques disclosed in U.S. Pat. No. 4,941,721.
Another problem, one common to both underfilled and overfilled designs, is fast scan jitter manifested by pixel misregistration in scan lines formed in the fast scan direction at the image medium. The main cause is rotational or velocity variations in the polygon scanner. Additional contributing factors of lesser importance are misalignment of optical components, scan direction failures and pixel clock circuit failures. The jitter problem is exacerbated in color ROS printers which require that successively formed color images be superimposed to form a composite color image. Image to image registration tolerances are in the order of .+-.50 .mu.m in the fast scan direction. Various prior art techniques are known to detect and correct fast scan jitter. U.S. Pat. No. 4,620,237 discloses a method for generating jitter correction signals by comparing peak to peak variations between integrated fast scan test signals. U.S. Pat. No. 4,429,218 discloses a scanning system which includes detection and signal comparison circuitry which enables synchronization of a scanning system relative to the power distribution within the scanning beam so as to align the information scanned on consecutive scan lines. Jitter correction is provided by varying the power level and focus of the scanning beam. U.S. Pat. No. 4,872,065 uses the Start of Scan (SOS) and End of Scan (EOS) sensors to determine an error time corresponding to a difference between a measured time interval and a reference time interval. Jitter is reduced by generating a dot recording clock signal frequency shifted in a predetermined direction. These prior art references are hereby incorporated by reference.
As mentioned above, the main factor contributing to jitter is velocity variations in the polygon scanner. In a paper by H. Horikawa, I. Sugisaki and M. Tashiro entitled "Relationship Between Fluctuation in Mirror Radius (within one polygon) and the Jitter", given at a SPIE meeting on Beam Deflection and Scanning Technologies in San Jose, Calif. from Feb. 25th to Mar. 1st 1991, and published in the proceedings of this meeting, the authors conclude that, in a polygon ROS system with an f-.theta. lens in the post-polygon optics (to focus the scanning beam in both directions), the only factor associated with the polygon geometry, contributing to jitter is the differential departure from flatness of the polygon facets. Stated alternatively, the magnitude of the jitter, in a polygon ROS with an f.theta. lens, is inversely proportional only to the uniformity of flatness of the polygon facets, from facet to facet. This conclusion also implies that a slight curvature in the facet surfaces does not give rise to jitter, if all the facets have the same curvature, but that a difference in curvature, from facet to facet, will cause jitter. It was perceived by the applicants that the magnitude of this effect was also inversely proportional to the uniformity of the illumination, in the fast scan direction, incident upon the polygon facet throughout its scan. By uniformity of the illumination is included the constancy of the location of the centroid of the illumination distribution relative to a coordinate system travelling with the facet, as well as the uniformity of the illumination distribution itself. That is, the more uniform the light incident on the polygon facet, in the fast scan direction, the smaller the magnitude of the jitter.
Thus, one method of reducing jitter is to make the illumination intensity distribution uniform in the fast scan direction at the polygon facet. An overfilled facet design presents a facet illumination requirement which differs from the underfilled design. Since the modulated beam has a Gaussian illumination intensity profile as the beam is expanded to overfill the facets, the amount of light reflected to the image medium varies because the polygon facets are sampling different portions of the Gaussian illumination intensity profile. One method of compensating for this facet-to-facet reflected light variation is to place an aspheric lens system in the pre-polygon optics and to change the Gaussian shape of the intensity profile of the modulated beams into a generally flat uniform intensity profile which overfills each facet. This technique is disclosed in U.S. Pat. No. 4,941,721. While not specifically discussing the effects on the reduction of jitter, this scanning system would inherently reduce jitter by creating a uniform intensity profile at the facets.
According to the present invention, two additional methods are provided for reducing jitter in an overfilled design. A first technique is to place a variable transmission filter in the optical path between the collimator lens which transmits the entire beam with its Gaussian illumination intensity profile and the polygon. The light transmission component smooths out or makes uniform the amount of light which is incident at the facet surface. The second technique is to use a collimator lens in the pre-polygon system which will transmit only a small portion of the output of a laser diode. The portion transmitted corresponds to the more uniform central portion of the beam with its Gaussian illumination intensity profile. More particularly, the present invention relates to an optical scanning system which reduces fast scan jitter in an overfilled polygon design comprising:
a source of high intensity modulated, polarized and collimated light beams having a Gaussian intensity profile, PA1 a polygon scanner having a plurality of light reflective facets interposed in the optical path between said light beam source and a light sensitive medium, PA1 optical means for fully and uniformly illuminating one of said facets to produce scanning beams which are reflected towards a light sensitive medium, while adjacent facets are at least partially illuminated and PA1 means for focusing said scanning beams reflected from said fully illuminated facet upon the surface of said light sensitive medium wherein said optical means includes means placed between said source and said scanner for varying the intensity profile of the collimated light beam.